The present invention generally relates to using a high temperature superconductor (HTSC) in a fault current limiter. More particularly, the present invention relates to a resistive fault current limiter which allows the HTSC to operate above its critical temperature. A particularly useful form of the invention relates to an HTSC fault current limiter which provides a higher faulted state resistance and a sharper increase in resistivity with increasing current than prior art devices.
A fault current is generally defined as a temporary and substantial increase in current carried by a power system. Such a fault, caused by equipment shorts and EMI induced current surges, can severely damage any electronic equipment associated with the faulted system.
Early attempts to address the problems associated with fault currents involved designing all equipment and instruments connected to a given power system to withstand an anticipated maximum fault current. Consequently, power systems, along with the related equipment and instruments, were over-designed and engineered, increasing the overall cost, weight, and size of such systems.
Where utilized, a fault current limiter is generally either a switched or unswitched device. With respect to the latter, current is commuted from a low impedance main path to a higher impedance shunt path by one of several available switching means. The fault current must be commuted within a short time (typically less than half the period of a 60 hertz cycle, i.e., less than about 8 milliseconds), necessitating synchronized operation and complex equipment and construction.
Unswitched devices of the prior art include those which have no impedance in the normal state, but exhibit an increase thereof when the current exceeds a specified threshold. Alternatively, unswitched devices have included various in-line fuse devices which change phase and generate high-arc voltages. While these unswitched devices of the prior art have demonstrated some utility, most are plagued with performance capability problems. In addition, all are prohibitive from an initial cost perspective and are expensive to maintain.
The search for an effective, efficient fault current limiter has been an on-going concern in the art. Most recently, such devices have incorporated superconductors, which change to a normal resistance state when the current exceeds a prescribed limit. Such devices of the prior art are designed to maintain the superconductor component under isothermal conditions, below the critical temperature. Such devices tend to have a low and often-times inadequate faulted resistance state and do not exhibit a sufficient increase in resistivity with increasing current.
The ideal characteristics of a fault current limiter have been described as:
1) Limitation of first peak of fault current; PA1 2) Low impedance and low energy losses in the normal state; PA1 3) No unacceptable harmonics in the normal state; PA1 4) Inherently automatic operation with no specialized detection and control devices; PA1 5) Smooth and gradual change of impedance, normal to fault and vice versa; PA1 6) Compactness; PA1 7) Fail safe operation; PA1 8) Modest cost.
It is therefore an object of the invention to provide an improved fault current limiter meeting one or more of the characteristics described above.
It is another object of the invention to provide a novel fault current limiter incorporating a high temperature superconductor operable under quasi-adiabatic conditions.
It is yet another object of the invention to provide an improved high temperature superconductor fault current limiter capable of producing a higher faulted state resistance.
It is a further object of the invention to provide an improved high temperature superconductor fault current limiter which provides a sharper increase in resistivity with increasing current.
It is another object of the invention to provide a high temperature superconductor fault current limiter such that resistivity is a function of superconductor size, whereby resistivity is a design parameter for the limiter device.
It is yet another object of the invention to provide a novel fault current limiter which is capable of automatically returning to a nonlimiting state after a fault current limiting operation.
It is yet another object of the invention to provide an improved fault current limiter having resistance controllable by selective cooling of a superconductor material.
It is a further object of the invention to provide a novel fault current limiter operable above its critical temperature during a fault current limiting operation.
It is a still further object of the invention to provide an improved fault current limiter operable in conjunction with a circuit breaker.
It is yet another object of the invention to provide a novel fault current limiter having its period of operation controlled by a conventional circuit breaker.
Other objects, features and advantages of the present invention will be readily apparent from the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings described below wherein like elements have like numerals throughout the several views.